Method of manufacturing silicon carbide whiskers

ABSTRACT

WHISKERS OF APPROXIMATE DESIRED DIAMETER ARE GROWN ON A UNIFORM ROUGH SURFACE OF A SUBSTRATE BY MEANS OF A NUCLEATING SUBSTANCE DUE TO THE PRESENCE OF CRYSTALLITES PROJECTING FROM THE SURFACE THE LATRAL DIMENSIONS OF   WHICH AT THE SURFACE APPROXIMATELY CORRESPOND IN SIZE TO THE THICKNESS OF THE CRYSTALS TO BE GROWN.

p 1972 w. F. KNIPPENBERG ETAL 3,692,478

METHOD OF MANUFACTURING SILICON CARBIDE WHISKERS Filed Feb. 2, 1970INVENTOR. W. F. KNIPPENBERG G.VERSPUI 0 v /Mk United "States I)atent @1hoe 3,692,478 Patented Sept. 19, 1972 9 1 Int. Cl. C01b 31/36, 21/06;Clllf 7/02 US. Cl. 423-345 2, Claims ABSTRACT OF THE DISCLOSURE Whiskersof approximate desired diameter are grown on a uniform rough surface ofa substrate by means of a nucleating substance due to the presence ofcrystallites projecting from the surface the lateral dimensions of whichat the surface approximately correspond in size to the thickness of thecrystals to be grown.

The invention relates to the manufacture of whiskers.

Whiskers are to be understood to mean herein crystals having a maximumaverage thickness of 100 ,um. the length of which is at least 10 timeslarger than the thickness.

As is known, whiskers can be formed by growing in a gaseous phase on asubstrate, a strong anisotropic crystal growth being promoted bynucleation of the substrate, that is to say such a treatment thatcrystallisation nuclei are formed on places on the substrate surfaceseparated from each other.

As is known, elongated crystals can be obtained in all kinds ofdimensions by using a substance, a so-called nucleant, which stimulatesthe formation of a local crystallisation nucleus. This is in particularthe case in the manufacture of crystal whiskers by VLS-growth.

VLS (Vapour-Liquid-Solid) crystal growth has already been described inTransactions of the Metallurgical Society of A.I.M.E. 233. (1965), 1053.In VLS crystal growth a substance to be crystallized (or its components)is taken up from a gaseous phase in drops of a substance nucleantlocally provided on a substrate and in which the substance to becrystallized is soluble, and said substance is deposited via the dropson the substrate in the form of elongated crystals. This method is, ofcourse, useful for growing whiskers and, as is known, has been appliedfor manufacturing whiskers of different substances.

For that purpose, a substrate is nucleated by sputtering orvapour-deposition of a substance, from which upon heating prior to orduring the crystal growth very fine drops are formed which can each beactive separately, as a liquid phase necessary for VLS-growth.

Cases of whisker growth are also known in which nucleation of thesubstrate surface takes place via the gas atmosphere. It is sometimesfound, as is also found in the case of VLS-growth, that the growncrystals, may comprise at their free end a spherical part which consistsat least partly of the nucleant used, so that it may be assumed that inthis case also VLS-growth has taken place, although, no VLS-growth hasbeen carried out deliberately.

The formation of a spherical part at the ends of the crystals may befound sometimes, even in whisker growth, in which a nucleant wasprovided on the substrate neither by sputtering or vapour-deposition,nor by the supply via the crystallisation atmosphere. It may be assumed,that in these cases also VLS-growth might have taken place, in whichcrystal nuclei have formed from impurities present in the startingmaterial for the crystals, in material from which the substrate or theapparatus are constructed and/ or in the gas atmosphere.

Moreover, there Were cases, in which either intentionally orunintentionally nucleation of the substrate has been produced with as asource the impurities present in the apparatus whereas no spherical partcontaining a nucleant was observed at the free end of the growncrystals. However, also in these cases the possibility of a nucleationand crystal growth mechanism as in VLS- growth need not be out of thequestion, because in cases in which VLS-growth has been identified insome of the crystals formed the characteristic spherical ends arelacking. This may be a result of evaporation and/or reaction in thecrystallisation atmosphere.

Finally the possibility exists that the nucleant only fulfils a functionat the beginning of the crystal growth and is certainly not active as aliquid phase in a VLS-growth during the whole crystal growth.

In all the above-described cases of crystal growth with intentional orunintentional nucleation of the substrate, it is required, in order toobtain a good yield of thin whiskers of slightly varying thicknesses,that the substrate should be densely populated with crystal nuclei ofequal size.

Of course, this requirement cannot be fulfilled as such if thenucleation of the substrate is produced by impurities accidentallypresent.

However, in many cases, also the intentional addition of a nucleant viathe gaseous phase does not open the possibility of controlling thethickness of the crystal, and often results in varying thicknesses andnon-uniform population of the substrate with crystallisation nuclei.

In those cases of forming whiskers in which VLS- growth has beenidentified and in which the substrate is nucleated with drops of anucleant, in the conventional manner of providing the drops, a dense anduniform population of drops of equal size cannot be realized as such.

For example, by powdering or smearing a substrate with a finely dividedsubstance of equal grain size, in a subsequent heating step a denseuniform population of the substrate with drops of equal size is notobtained as such in all circumstances. Actually, upon heating, powderedparticles which are in contact with each other may flow together, sothat in addition to drops which correspond in volume to that of theprovided grains, larger drops will also be formed. If nucleation iseffected by vapourdeposition of a substance, suitable for VLS-growth,followed by heating, the vapour-deposited layer will divide inindividual drops but usually the sizes of these drops will vary and,moreover, may form an irregular, poorly dense pattern of crystallisationnuclei on the substrate.

It is an object of the present invention to avoid the above-mentioneddrawbacks associated with the nucleation of substrates for growingwhiskers.

The invention relates to a method of growing crystals in the form ofwhiskers on a substrate in a gaseous atmosphere containing the substanceto be crystallized or its components, in which strong anisotropiccrystal growth is stimulated by nucleation of the substrate,characterized in that the crystals are grown on a substrate which isrough at least superficially due to the presence of crystallitesprojecting from the surface the lateral dimensions of said crystallitesat said surface approximately corresponding in size to the thickness ofthe crytsals to be grown.

As a result of this uniform rough structure of the substrate surface itis achieved that, independent of the fact whether the nucleatingsubstance is supplied in a solid, liquid or vapour form, a dense anduniform nucleation of the substrate will always take place.

The nucleant may be applied in an elementary form but in that casevariations in the choice of the crystallisation circumstances inconnection with the volatility of the nucleant, that is to say in thechoice of the temperature, the partial pressure of the substance to becrystallized or its components and the overall pressure of the gasatmosphere, are limited.

In many cases, however, it is of advantage to supply the nucleant in theform of a compound which can be converted into a nucleant bydisproportioning or other chemical reaction because in that case varioussubstances of a varying volatility are available as a result of whichthere is a greater possibility of adapting the crystallisationconditions in a correct manner.

Also for replenishing the nucleation via the gas atmosphere during thecrystal growth, in so far as the nucleant, necessary for the crystalgrowth, is fully or partly lost by side reactions or evaporation anddissipation in the gas atmosphere, it is of advantage, for the samereasons as mentioned above, to use the nucleant in the form of acompound which can be converted into a nucleant by disproportioning orby the other chemical reaction. D

In the cases in which the nucleation of the substrate is carried out viathe gaseous phase, nothing can be established visually as regards thedensity and uniformity of the nucleation of the polycrystallinesubstrate. Only the uniformly dense growth of whiskers of slightlyvarying thickness can be established. I

Clearly visible, however, is the density and uniformity of the moremacroscopic nucleation as is used in intentional VLS-growth.

When the nucleant or a compound which can be converted into a nucleantis provided on the substrate by vapour-deposition or by sputtering, theformed continuous layer will be divided into separate drops upon heatingand form a dense uniform pattern of drops Wind: is determined by theuniformly rough structure of the substrate surface.

This is the case also when the nucleant or a compound thereof isprovided on the substrate in a finely divided form by spraying orsmearing. Upon heating, mainly a pattern of drops is formed which isdetermined by the lateral dimensions of the crystallites in the plane ofthe substrate surface. This result is even promoted by providing thenucleant in the form of powder of slightly varying grain size andparticularly having a gram size which corresponds at most to the saidtransverse dimensions of the crystallites.

Of course the substrates should be constructed from a material which isresistant to the gas atmosphere in which and the temperature at whichthe crystal growth is carried out.

Substrates which consist at least superficially of polycrystallinematerial of the above crystallite size can be manufactured bysublimation or by gas reactions in which, of course, parameters, such asvapour pressure and temperature, should be suitably chosen so as also toobtain the required roughness.

Substrates are preferred, however, which consist of powder agglomerateswhich can be obtained by sintering, pressing or adhering with a binderin any suitable form, since in the manufacture thereof the crystallitedimensions and the surface roughness can easily be controlled by thechoice of the grain size and the grain shape of the starting material.

If, however, there has been proceeded so that a porous material isobtained, this provides the additional possibility that the supply ofthe nucleant and of the substance to be crystallized or its componentsmay be carried out via a gaseous phase through the substrate.

Finally the manufacture of cohering powder agglomerates provides afavourable possibility of forming nucleated substrates in one operationby starting from a powdered substrate material, in which the nucleant ora compound which can be converted to a nucleant is distributed evenly inthe form of a powder having a grain size corresponding to the thicknessof the crystals to be formed.

A particular advantage of powder agglomerates as substrates is that itcan simply be obtained by means of a suspension of the raw materials ona support.

The invention will now be described in greater detail with reference tothe accompanying drawing and a few specific examples.

EXAMPLE 1 A plate of 25 x x 2 mm. is formed by sintering silicon carbidepowder having a grain size of 1 am. at 2200 C. for 30 minutes. The plateis then atomized on one side with iron powder having a grain sizesmaller than 1 ,am. and the excessive iron powder is wiped oif.

As is shown in the diagrammatic cross-sectional view of FIG. 1 of theaccompanying drawing, a substrate 1 thus formed which is nucleated withiron powder 2 in a graphite crucible 3 is placed on a bed of quartzpowder 4.

The assembly is arranged in a quartz tube 5, through which hydrogen isled at a rate of 1 litre per minute and heated at 1280 C.

Whiskers of silicon carbide, diameter 1 m., length 2 cm., grow on thesubstrate in 10 hours. The yield is mgms. of whiskers per plate.

EXAMPLE 2 Aluminum oxide powder (pro analysis) having a grain size of0.01 to 0.1 ,um. is mixed with ethyl acetate and compressed to tabletsof 10 x 100 x 1 mm.

As shown in the diagrammatic plan view of FIG. 2, a few of these tablets11 serving as substrates and aluminum oxide crucibles 12 filled withaluminum 13 are placed on a bed of quartz grains 14 in an aluminum oxideboat 15. The assembly is surrounded by a tube of aluminum oxide 16through which a hydrogen current of 1 litre per minute is led, andheated by means of an oven at 1300 C. The substrate is nucleated via thegaseous phase with aluminum.

Ribbon-shaped aluminum oxide whiskers formed from Al oxidised by SiOgrow in 6 hours and have a width smaller than 0.1 ,um., a thickness of0.01 m, and a length of 2 cm., which at their end, have an Al head.

The substrate is dense and uniformly populated with whiskers. The yieldis 200 mgms.

EXAMPLE 3 The substrate used in this example is a commercial productconsisting of a graphite plate provided with a grown pyrographite layerof known orange peel structure having projecting graphite cones ofapproximately 10 am. diameter (Chernie, Ingenieur Technik, nr. 39, vol.14 (1967), 833).

A plate of 100 x 20 mm. of this material is smeared with powder ofcarbonyl nickel having a grain size smaller than 1 m.

As shown in the cross-sectional view of FIG. 3, this plate 21 nucleatedwith nickel is laid on a graphite crucible 22 as a cover in whichcrucible quartz powder 23 is present. The assembly is arranged in a tube24 of quartz and a flow of 75% nitrogen with 25% hydrogen is led throughat a rate of /2 litre per minute. By high frequency heating of thecrucible for 60 hours at a temperature of 1250" C., whiskers of siliconnitride, diameter 10 m, length 10 mm., grow on the lower side of thesubstrate 21 in a uniformly dense population.

EXAMPLE 4 Onto a quartz plate the surface of which is provided with anadhesive layer consisting of a rubber adhesive, a powdered mixture ofsand and gold in a weight ratio of 1:10 having a grain size of 30 m isspread and nonadhered powder particles are flicked off. In this manner amonograin layer of sand and gold is formed on the quartz plate.

The resulting substrate nucleated with gold is arranged in a quartztube.

A gas-current consisting of hydrogen with 2 mol percent SiCL of 20litres per hour is led through.

Upon heating at 1050 C. for 60 hours a uniformly dense growth of siliconwhiskers having a length of 1 cm. and a diameter of 30 m. is formed onthe substrate.

EXAMPLE 5 Zirconium oxide powder, having a grain size 1 m is mixed with1% by weight of Fe O grain size 0.1 m. A suspension of the mixtureobtained is spread on a quartz plate in the form of a thin layer.

The resulting substrate is arranged in a quartz tube, and a gas currentconsisting of hydrogen with 3% propane is led through said tube at arate of 1 litre per min.

The iron oxide is converted in a uniformly dense nucleation of iron bythe hydrogen upon heating at 1280 C. Furthermore, silicon monoxide isdeveloped from the quartz so that a crystallisation atmospherecontaining silicon and carbon is formed, as a result of which whiskersof silicon carbide, having diameters of 0.1 ,uIIL, lengths of 1 cm.,grow at the locations of the iron nuclei.

What is claimed is:

1. A method of growing crystals in the form of whiskers which comprisesforming a substrate by sintering silicon carbide powder having a uniformgrain size at a temperature of about 2200 C. to form a plate thereof;atomizing the plate on one side with iron powder to nucleate it, saidiron powder having a grain size which is smaller than the siliconcarbide powder, and wiping ofl any excessive iron powder from the plate;placing the thus formed substrate in a graphite crucible resting on abed of quartz powder, arranging the crucible with the substrate restingon the quartz powder in a quartz tube, and flowing hydrogen gas heatedto a temperature of 1280" C. through the quartz tube; whereby whiskersof silicon carbide having a diameter of 1 micrometer and a length of 2centimeters are grown on the substrate in response to strong anisotropiccrystal growth stimulated by the nucleated substrate.

2. A method as claimed in claim 1, wherein the hydrogen flow is oneliter per minute.

References Cited UNITED STATES PATENTS 8/ 1965 Merkel et a1. 23223.5 5/1971 Blakeslee et a1 148--1.6

U.S. Cl. X.R. 23-625; 423-400 mg UNITED STATES PATENT oTTTcE ERTiFlATEOF QGRREQTWN Patent No. 3692478 Dated September 19 1972 Inventor(s)WILHELMUS FRANCISCUS KNIPPENBERG ET AL It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

-1 In the heading: before "Filed Feb. 2, 1970" insert assignors to U. S.Philips Corporation,

New York, N. Y.

Col. 3, line 1, "temperature" should read temperatures Signed and sealedthis 10th d f April 1973,

(SEAL) Attest:

EDWARD 1 I.PLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

